1. Field of the Invention
The present invention relates to a hologram recording/reproducing device and a recording/reproducing optical apparatus using the hologram recording/reproducing device.
2. Description of the Related Art
A hologram recording device and a hologram recording method, used for recording data using holography, are proposed. In the device and method, reference light and signal light, modulated by information to be recorded (record data), are generated by the same laser light source, and are used to irradiate a hologram recording/reproducing medium. This causes the signal light and the reference light to interfere with each other at the hologram recording/reproducing medium to form a diffraction grating (hologram) in the hologram recording/reproducing medium, so that the record data is recorded in the form of the diffraction grating (hologram).
A hologram reproducing device and a hologram reproducing method, used for reproducing the record data from the diffraction grating (hologram) recorded in this way, are proposed. In the device and method, when the diffraction grating (hologram) formed on the recording medium to which the recording has been performed is irradiated with the reference light, diffraction light (that is, reproduction light) is generated. The reproduction light is detected by a light-receiving element to reproduce the record data.
Two types of recording/reproducing methods are proposed for generating the signal light and the reference light in such a recording operation and a reproducing operation. They are a two-beam interference recording/reproducing method and a collinear recording/reproducing method. In the two-beam interference recording/reproducing method, a path of the signal light and a path of the reference light are completely separately provided, whereas, in the collinear recording/reproducing method, the signal light and the reference light are disposed on the same axis and share one path. In the collinear recording/reproducing method, a reference-light pattern is formed at an outer peripheral portion of a spatial-light modulator (SLM) and a signal-light pattern is formed at an inner peripheral portion of the spatial-light modulator to record them on a recording medium. In addition, only the reference-light pattern is formed at the outer peripheral portion of the spatial-light modulator, and reproduction light is obtained from the recording medium to which the recording has been performed to reproduce record data (refer to, for example, U.S. Pat. No. 6,108,110 and Nikkei Electronics (P 106 to 114 of the Jan. 17, 2005 issue).
A recording/reproducing optical unit 150, which is a main portion of a recording/reproducing device that performs a recording operation and a reproducing operation, is shown in FIG. 9. A transmissive collinear recording/reproducing method will be simply described with reference to the recording/reproducing optical unit 150 shown in FIG. 9.
Information is recorded in the following way. A light beam emitted from a laser light source 101 for recording and reproducing the information is incident upon a spatial-light modulator 103 through a collimator lens 102. The spatial-light modulator 103 spatially divides the light beam into light-beam portions, one portion becoming signal light 108 having a light-intensity modulation pattern reflecting record information and the other portion becoming reference light 109 which is not subjected to light intensity modulation or which is subjected to a particular light intensity modulation. These portions of the light beam reach an objective lens 104. For the spatial-light modulator 103, for example, a combination of a polarizing plate and an array liquid crystal panel is used. Since the signal light 108 and the reference light 109 pass through the same objective lens 107, the recording/reproducing method is called the collinear recording/reproducing method. The objective lens 104 causes the signal light 108 and the reference light 109 to form an interference fringe, that is, a hologram, at an information recording layer in a transmissive hologram recording/reproducing medium 307.
The information is reproduced in the following way. A light beam emitted from the laser light source 101 is transmitted through the collimator lens 102 and reaches the spatial-light modulator 103. The signal light 108 generated from the light beam is blocked by the spatial-light modulator 103 whose transmissivity is controlled to 0%, so that only the reference light 109 illuminates the recorded hologram through the objective lens 104. The light beam that has been diffracted by the hologram in the transmissive hologram recording/reproducing medium 307 passes through a condenser lens 105 to form a reproduction image on an array light detector 106 and to detect a spatial distribution of light intensity of the reproduction image by the light detector 106. Here, the light detector 106 is an array light detector, such as a CCD sensor or a C-MOS sensor.
In the collinear recording/reproducing method, the recording/reproducing optical unit may be a reflective type. FIG. 10 shows an example of a structure of a reflective collinear recording/reproducing optical unit 151. Parts of the optical unit 151 having structural features and functions that are similar to those of the optical unit 150 shown in FIG. 9 will be given the same reference numerals and will not be described below. In a reflective collinear recording/reproducing method, a reflective hologram recording/reproducing medium 207 having a reflective film at the back of an information recording layer is used. Recording of information in the reflective type is substantially the same as that in the transmissive type. A difference is that signal light 108 and reference light 109 pass through a beam splitter 110 to form a hologram at the information recording layer in the hologram recording/reproducing medium 207 by the objective lens 104. The information is reproduced as follows. A light beam that has been diffracted and reflected by the hologram in the hologram recording/reproducing medium 207 passes through the objective lens 104 again and is reflected by the beam splitter 110, so that a reproduction image is formed on an array light detector 106 and a spatial distribution of light intensity of the reproduction image is detected by the light detector 106.
FIG. 11 shows an example of a pattern disposed at the spatial-light modulator 103 for splitting the signal light 108 and the reference light 109 transmitted through the spatial-light modulator 103 of the hologram recording/reproducing device. In general, a signal-light area 118 for generating the signal light 108 is disposed at an inner peripheral portion of the spatial-light modulator 103 where good optical performance is provided, a reference-light area 119 for generating the reference light 109 is disposed at an outer peripheral portion of the spatial-light modulator 103, and a gap is provided between the signal-light area 118 and the reference-light area 119.
To record a large amount of information on a hologram recording/reproducing medium, what is called multiplex recording for forming a plurality of holograms at one location (or overlapping areas) of the hologram recording/reproducing medium may be performed. Various multiplex recording methods are proposed (refer to, for example, Nikkei Electronics (P 106 to 114 of the Jan. 17, 2005 issue)).
FIG. 1 shows a result of simulation of light intensity distributions near a focal plane of the signal light 108 and a focal plane of the reference light 109, in a range of a recording-surface cross section (that is perpendicular to a recording/reproducing surface) of the hologram recording/reproducing medium 207. In FIG. 1, dark-colored portions A indicate main portions of the reference light 109, and a light-colored portion B indicates a main portion of the signal light 108. Here, the reflective collinear recording/reproducing device shown in FIG. 10 is used as a recording/reproducing device, and the spatial modulation pattern shown in FIG. 11 is used. When the recording/reproducing device and the spatial modulation pattern according to the related art are used, as is clear from FIG. 1, the reference light 109 and the signal light 108 interfere with each other only at a small area near the focal planes of the signal light 108 and the reference light 109 (that is, the lower portion at the center in FIG. 1).
In other words, FIG. 1 shows that a hologram is only formed at the small area near the focal planes in a hologram medium. In general, a hologram medium having a thickness on the order of from 0.5 mm to 1 mm is used, on the basis of the expectation that, if the thickness of the medium increases, the number of multiplexings is increased, so that recording density is increased. As is clear from the simulation result shown in FIG. 1, the feature expected of hologram recording that increasing the thickness of the medium increases the number of multiplexings and, thus, the recording density is not made use of. Therefore, the information recording layer disposed in the hologram recording/reproducing medium 207 is not effectively used. In other words, that the thickness of the medium substantially does not contribute to increasing the recording density and the information recording layer is wastefully used is shown in FIG. 1.
Accordingly, it is desirable to provide a hologram recording/reproducing device which can considerably increase an area of interference between reference light and signal light in a medium as compared to that in a related method, to solve such a problem, and which does not wastefully use an information recording layer, that is, which effectively makes use of the information recording layer to increase recording density. It is also desirable to provide an optical apparatus using the hologram recording/reproducing device.